The present invention is a limiting circuit for use with an amplifier having high direct-current (DC) stability and high immunity to oscillation, the amplifier being commonly referred to as an operational amplifier or opamp. The opamp is commonly used with negative feedback circuitry to perform analog operations such as summation and integration.
Heretofore, it has been recognized that operational amplifiers require overload protection or limiting circuitry to avoid damage to the opamp and to eliminate abnormal or unpredictable behavior within the circuit which utilizes the opamp when it is driven into saturation. While the opamps are intended to be used over a voltage range where a linear response is found, it may, in certain situations, be desirable or necessary to over-drive them into saturation. Unfortunately, when the opamp is driven into saturation there is no feedback to maintain the input differential at zero volts. Under saturation conditions, the inputs may be driven to "abnormal" potentials, thereby charging up input capacitances, etc., which in turn creates recovery problems when the input signal is reduced, due primarily to hysteretic behavior.
Some of the approaches used to protect or limit operational amplifier feedback circuits are illustrated in the following disclosures which may be relevant:
U.S. Pat. No. 3,586,991 PA0 Patentee: Vosteen PA0 Issued: Jun. 22, 1971 PA0 U.S. Pat. No. 4,429,282 PA0 Patentee: Saari PA0 Issued: Jan. 31, 1984 PA0 Publication: Xerox Disclosure Journal PA0 Author: Milkie et al. PA0 Volume 12, No. 2, pp. 85-86 PA0 March/April 1987
The relevant portions of the foregoing patents and disclosure may be briefly summarized as follows:
U.S. Pat. No. 3,586,991 to Vosteen discloses an overload protection circuit for an amplifier, where the operating potential of the amplifier is limited to a potential less than the destruction potential of the amplifier. Zener diodes are used in an amplifier protection circuit to prevent the application of voltages capable of damaging the elements of the amplifier.
U.S. Pat. No. 4,429,282 to Saari discloses a high performance operation amplifier circuit which nulls the offset voltage by means of switched capacitors, and holds the output signal during nulling. A pair of non-overlapping pulse trains are used to control the switched capacitor nulling circuit which requires no filtering in the feedback path. Also disclosed is an offset limiting feedback network for clipping the output at a level inside the limits of the amplifier, thereby preventing the locking of the circuit which may result from an excessive initial offset voltage.
The disclosure in the Xerox Disclosure Journal by Milkie et al. (Volume 12, No. 2, pp. 85-86) teaches a high voltage stabilization circuit that may be used in applications for corona charging. The circuit employs a pair of zener diodes, working in conjunction with a pair of operational amplifiers to, among other things, create a dynamic error offset which is proportional to the desired high voltage potential and maintain the desired output with overload protection.
The present invention is a device for dynamically limiting the gain of a circuit employing an operational amplifier, a circuit which may have input signals which would normally over-drive the amplifier's output into a saturation condition. The advantage of the circuit is that the limiting capability dynamically reduces the gain such that the amplifier is never driven into saturation and, therefore, does not need to recover from what would otherwise be a saturation condition.
In accordance with the present invention, there is provided a dynamically limited amplifier including an operational amplifier having a pair of input terminals with an input potential therebetween, an output terminal, and a transistor element connected between one input terminal and an output terminal of the operational amplifier. The transistor element becomes operational and supplies a feedback current between the output terminal of the operational amplifier and the input terminal of the operational amplifier, whenever the output voltage of the operational amplifier exceeds a threshold potential to substantially prevent saturation of the operational amplifier.
In accordance with another aspect of the present invention, there is provided a null-seeking AC carrier feedback system, where the AC input signal passes through a zero potential. The AC carrier feedback system includes an operational amplifier, a feedback resistor connected between an inverting input of the operational amplifier and the output of the operational amplifier, and an input resistor connected between the AC input signal source and the inverting input of the operational amplifier. The AC carrier feedback system also includes a dynamic limiter which is connected between the inverting input and the output of the operational amplifier. The dynamic limiter includes an NMOS field-effect transistor having a drain connected to the inverting input of the operational amplifier, a source connected to the output of the operational amplifier, and a gate connected to a first reference potential, so that when the operational amplifier output potential is less than a negative threshold, determined as a function of the first reference potential, the NMOS field-effect transistor will operate in an enhancement mode to provide a feedback current to the input terminal of the operational amplifier. The dynamic limiter also includes a PMOS field-effect transistor having a drain connected to the inverting input of the operational amplifier, a source connected to the output of the operational amplifier, and a gate connected to a second reference potential, so that when the output terminal potential exceeds a positive threshold, determined as a function of the second reference potential, the PMOS field-effect transistor will operate in an enhancement mode to provide a feedback current to the input of the operational amplifier.